When you press the brake pedal, and brake piston pushes the pads into the rotor.
But, there is no spring that makes the pads pull away from the rotor.
So, what makes them pull off the rotor?
I assume they don’t just stay pressed against the rotor and keep scraping…
With disk brakes, there is no mechanism to “recoil” them. With brake pedal pressure removed, the pads intentionally just ride there and “self clean”. The resulting friction is negligible.
The answer is a square cut seal, that helps pull the pads back just a hair but its not visible to the naked eye.
WheresRick:
I had forgotten about the square cut seal. I guess I was thinking the OP wanted an answer similar to how springs in drum brakes retract the shoes. I stand corrected.
But to be clear, that square cut seal acts as an “assist” because it simply wants to go from a tilt position (when fluid pressure is on) back to it’s relaxed position. Disk brakes are intentionally self-cleaning, so it’s really not an act of pulling the pad/piston away from the rotor.
Were both right, both statements are true,
Yes to all above, plus, rotors always have some wobble due to lack of pre-loading the bearings and the wobbling throws the pads out, overcoming the compression from the caliper slides and/or piston friction.
The square cut seal may explain the piston, but it doesn’t explain the pad on the other side. With the brake pedal depressed, hydraulic pressure holds the brake pads tightly to the rotor. It takes a lot of pressure to hold them there, that is why disc brakes have power assist.
Foot off the pedal and the pads barely touch the rotor. If there is any wobble in the rotor, then the brake pads will separate a little further, but ideally there is always a little drag. This one reason that NASCAR team owners avoided using disc breaks early on, the drum brakes without self adjusters and adjusted with a little extra clearance had less drag and therefore higher top speeds.
Keith,
My race car has no power assist and has plenty of pressure. The key to this is the ratio of the master cylinder to the slave (brake end) cylinder.
The seals and dust boots tend to retract the caliper piston slightly, but the slight amount of play in the wheel bearings allow the rotors to wobble enough to push the pads back enough so there is virtually no friction…
CapriRacer, how much does your race car weigh? That has a lot of bearing too. Even my 68 Datsun SSS had power assist for the disc brakes as did my wife’s Chevy Sprint (later Geo Metro).
I have “manual” discs I put on a 71 Chevelle with around 575 HP. It stops just fine without a booster. Its not for some frail old lady though. 
Also some vintage 60s vettes but they would be in the “lighter” category. The Chevelle has all the original metal…
Could one of you folks explain the “square cut seal” concept to me? Thanks! Rocketman
keith,
It doesn’t matter what the vehicle weighs. Sizing brake systems is pretty complex, but It’s just easier to do if you have the advantage of a power system - particularly if cost is an issue. It’s easier, but not necessary.
On my race car, I have about twice the braking power my stock system had - even with the additional power assist. But that does come with certain compromises - one is that the brake pads have to heat up before they work well - something totally unacceptable in a street car.
One additional mechanism not yet mentioned. Although some square-cut seals pull the pads completely free of the rotors, on most cars, after the square-cut seal pulls the pads back a bit, there is still noticeable drag when you spin the wheel. That drag would generate considerable heat and wear the pads if not for this last mechanism.
At highway speed, there is a boundary layer of air a few thousandths of an inch thick spinning with the rotor. The disk brake pads ‘fly’ on this boundary layer of air, and don’t touch the rotor at highway speed.
Rocketman; The caliper to piston seal is a square “O” ring instead of a round “O” ring. It has more sealing area than a round sealing ring so it seals better in this application. It has little or nothing to do with retracting the caliper piston after brake application. As I mentioned before, it’s the play in the wheel bearings that push the piston back a little…
To demonstrate this, jack a front wheel off the ground. Step on and release the brake. The wheel will now have considerable drag. Lower the car and turn the steering wheel back and forth a few times. Jack wheel off floor and check for brake drag again. There will be very little…The slight play in the wheel-bearings allowed the ROTOR to push the caliper piston(s) back enough to eliminate brake drag…
A little clarification here, the cross section of the O-ring is a square, not the shape of the O-ring.
With all due respect to @Caddyman, the square cross section does indeed retract the brake caliper pistons. I used to work for GM’s Delco and the brake engineers told me that is how they retract the pads to reduce friction and increase fuel economy. The slight wobble in the rotor due to warp and bearing slop centers the pads on sliding caliper designs for the least friction. The downside to all this extra clearance is that the brake pedal has to move farther to get the initial brake apply. They did all kinds of silly things like stepped-bore master cylinders and such to take up the initial slop. You can also use a bigger master cylinder bore (which applies less pressure) with a greater boost ratio (for increased force) from the power brake booster to compensate.
If the sealing rubber did anything, the brakes would fully release with the wheel jacked up and the brake applied and released…But that is not the case, the brake will have considerable drag until you whack the wheel/tire with a heavy rubber hammer which will jar the rotor enough to push the piston back a few thousandths and release the brake completely…
The caliper piston seal is of a “square cut” design. The outside edge of the groove (the edge of the groove nearest the inner brake pad) is cut at a slight angle. This angle allows the seal to flex slightly on brake application.
Then when the brake is released the seal then returns to its normal shape pulling the piston back to its original position. If the pads have worn enough, the seal will actually make contact with the angled edge and stop. The piston will then continue to move, slipping on the seal allowing the piston to adjust for wear.
The seal does retract the piston.
If your car needs the run out of the rotors to fully release the brakes, you have a binding slide or something…